Field of the Invention
The present invention generally relates to guidance and control systems and, more particularly, to guidance and control systems for an expendable store.
Description of the Related Art
Tactical fighter and bomber aircraft have been used to carry and deliver ordinance since shortly after the dawn of aviation. In the earliest stages of air combat, separation of stores from the parent aircraft was of little concern. However, during the Vietnam War, the employment of heavy stores from larger jet-powered aircraft began to present difficulties for aircraft-store compatibility. Specifically, scenarios in high-speed flight were encountered where the released store failed to separate cleanly from the aircraft and instead became a projectile threatening the aircraft and on occasion re-contacting the aircraft in flight causing catastrophic damage and loss of life.
A store released from an aircraft in flight must traverse a nonuniform and unsteady flow field that may include complex shock interactions, large velocity gradients, regions of locally separated or reversed airflow, and severe flow angularity in the form of sidewash and downwash. Stores released from an internal weapons bay may also be subjected to a wake disturbance from the spoiler, dynamic pressure and velocity gradients across the shear layer, high frequency vibrations due to acoustic noise, and large perturbations in flow properties due to cavity oscillations. Although the region of nonuniform flow near the aircraft is exceedingly small compared to the full length of the store ballistic or fly-out trajectory, the effects are significant.
Store separation engineering, a subset of aircraft-store compatibility, is concerned with the flight characteristics of a store in proximity of the aircraft and other stores. Ground test, flight test, simulation, and analysis procedures have been developed which largely address the safety-of-flight issues first encountered in the Vietnam era. In most cases, the store can be ejected away from the aircraft with a sufficient vertical velocity and nose-down pitch rate to ensure safe separation. However, with the advent of smart weapons, standoff capabilities, and focused lethality the challenge in successful store separation has shifted from safety to acceptability. Whereas an unsafe separation may threaten the parent aircraft, an unacceptable separation may result in a failed mission or significant collateral damage due to guidance problems, loss of control, or damage to the store caused by the separation transients.
Modern sophisticated “smart” weapons are equipped with sensitive onboard electronics including inertial measurement systems, GPS units, sensors, seekers, and guidance computers. Standoff capability (the desirable ability to release a munition far away from the intended target) has resulted in complex aerodynamic shapes with neutral dynamic stability margins designed for maximum glide performance and minimal energy loss. Focused lethality (the desirable ability to destroy a designated target while minimizing collateral damage) has resulted in munitions that are smaller and lighter and therefore more dramatically affected by the exigent flow field surrounding the aircraft in flight. These tendencies have increased the sensitivity to separation-induced transients, potentially leading to large angular rates and attitudes, excessive energy loss, sensor saturation, structural limits, or departure from stable flight modes. A challenge in store separation is thus to ensure safety while also maintaining acceptability across the flight envelope.
Modern munitions are designed with an onboard guidance and control system to enable precise engagement of the intended targets. However, the control system is not usually activated until the store is sufficiently far away from the aircraft to avoid any potential interference. Often, the separation-induced transients result in large perturbations from the desired flight attitudes that require a dedicated “rate-capture” phase for recovery before the munition can begin the fly-out trajectory. In the relatively few cases where the autopilot is engaged earlier (to prevent build-up of irrecoverable rates and attitudes), the mutual aerodynamic interference between the store and aircraft is neglected in the autopilot design leading to increased risk through reduced confidence in simulation capabilities and potentially unsafe behavior of the autopilot reacting to flow field perturbations without consideration of the nearby aircraft.
Accordingly, there is a need in the art for a transitional control system that accounts for separation-induced transients to guide a store along a preferred trajectory.